L1.2_Slides
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Transcript of L1.2_Slides
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CONCORDIA
VLSI DES IGN LAB
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Fabrication ProcessCrystal Growth
DopingDeposition
Patterning
Lithography
Oxidation
Ion Implementation
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Fabrication- CMOS Process
Starting Material Preparation
1. Produce Metallurgical Grade Silicon (MGS)
SiO2 (sand) + C in Arc FurnaceSi- liquid 98% pure
2. Produce Electronic Grade Silicon (EGS)
HCl + Si (MGS)Successive purification by distillationChemical Vapor Deposition (CVD)
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Fabrication: Crystal Growth
Czochralski Method
Basic idea: dip seed crystal intoliquid pool
Slowly pull out at a rate of0.5mm/mincontrolled amount of
impurities added to melt
Speed of rotation and pullingrate determine diameter of theingotIngot- 1to 2 meter longDiameter: 4, 6, 8
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Fabrication: Wafering
Finish ingot to precise diameter
Mill flats Cut wafers by diamond saw:
Typical thickness 0.5mm
Polish to give optically flatsurface
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Fabrication: Oxidation
Silicon Dioxide has several uses:- mask against implantor diffusion
- device isolation- gate oxide- isolation between layers
SiO2 could be thermally generated or through CVD
Oxidation consumes silicon
Wet or dry oxidation
Quartz Tube
Wafers
Quartz Carrier
Resistance Heater
O2or Water
VaporPump
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Fabrication: Diffusion Simultaneous creation of p-n junction over
the entire surface of wafer Doesnt offer precise control
Good for heavy doping, deep junctions Two steps:Pre-depositionDopant mixed with inert gas introducedin to a furnace at 1000 oC.Atoms diffuse in a thin layer of Si surfaceDrive-inWafers heated without dopant
Resistance Heater
wafers
Temp: 1000
Dopant Gas
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Fabrication: Ion
ImplantationPrecise control of dopantGood for shallow junctions and threshold adjustDopant gas ionized and acceleratedIons strike silicon surface at high speed
Depth of lodging is determined by accelerating field
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Fabrication: Deposition
Reactant0.1 -1 Torr
Loader
Pump
Used to form thin film of Polysilicon,Silicon dioxide, Silicon Nitride, Al.
Applications: Polysilicon, interlayeroxide, LOCOS, metal.
Common technique: Low PressureChemical Vapor Deposition (CVD).
SiO2 and Polysilicon deposition at300 to 1000 oC. Aluminum deposition at lower
temperature- different technique
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Fabrication:
Metallization Standard material is Aluminum
Low contact resistance to p-type and n-type
When deposited on SiO2, Al2O3is formed: good adhesive
All wafer covered with Al
Deposition techniques:
Vacuum EvaporationElectron Beam EvaporationRF Sputtering
Other materials used in conjunction with or replacement to Al
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Fabrication: EtchingWet Etching Etchants: hydrofluoric acid (HF), mixture of nitric acid and
HF Good selectivity Problem:
- under cut- acid waste disposal
Dry Etching Physical bombardment with atoms or ions good for small geometries. Various types exists such as:
Planar Plasma EtchingReactive Ion Etching
Plasma Reactive species
RF
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Fabrication:
LithographyMask making
Most critical part of lithography is conversion from layout tomaster mask
Masking plate has opaque geometrical shapes correspondingto the area on the wafer surface where certain photochemicalreactions have to be prevented or taken place.
Masks uses photographic emulsion or hard surface
Two types: dark field or clear field
Maskmaking: optical or e-beam
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Fabrication:
Lithography
Step & Repeat
Printing
Printing
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Lithography: Mask
makingElectron Beam Technique
Main problem with optical
technique: light diffraction
System resembles ascanning electron microscope +
beam blanking and computercontrolled deflection
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Patterning/ PrintingProcess of transferring mask features to surface of the silicon wafer.
Optical or Electron-beam
Photo-resist material (negative or positive):synthetic rubber orpolymer upon exposure to light becomes insoluble ( negative ) orvolatile (positive)
Developer: typically organic solvant-e.g. Xylen
A common step in many processes is the creation and selectiveremoval of Silicon Dioxide
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Patterning: Pwell mask
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Patterning/ Printing
substrate
SiO2
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Fabrication Steps
Apply PR
Pre-bake
Printer align exposemask
Develop, rinse, dry
Post bake
Inspect, measure
Etch
Strip resist
Deposit or grow layer
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Fabrication Steps
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Fabrication Steps: P-
well ProcessDiffusion
VDD
Vo
P well
P+
P+
n+
n+
Vin
p+
P well
p+p+ p+n+ n+
Substrate n-type
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Fabrication Steps: P-
well ProcessDiffusion
VDD
Vo
P well
P+
P+
n+
n+
Vin
p+
P well
p+p+ p+n+ n+
Substrate n-type
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Fabrication Steps
n+n+
P well
p+ p+
Substrate n-type
P well
n+ n+p+ p+
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Fabrication StepsOxidation
Substrate n-type
Patterning of P-well mask
Substrate n-type
oxide
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Fabrication StepsDiffusion: p dopant,Removal of Oxide
Deposit Silicon Nitride
P-well
P-well
Si3N4
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Fabrication StepsPatterning: Diffusion (active)mask
Oxidation
P-well
substrate
FOXFOX FOX
substrate